Microwave plasma generating devices and plasma torches

ABSTRACT

The invention relates to a plasma generating device that comprises at least one very high frequency source (&gt;100 MHz) connected via an impedance adaptation device to an elongated conductor attached on a dielectric substrate, at least one means for cooling said conductor, and at least one gas supply in the vicinity of the dielectric substrate on a side opposite to that bearing the conductor. The invention also relates to plasma torches using said device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.12/679,231, filed Sep. 16, 2008, which is a 371 of International PCTApplication PCT/FR2008/051659, filed Sep. 16, 2008, which claimspriority to French Patent Application No. 07 57719, filed Sep. 20, 2007,the entire contents of which are incorporated herein by reference.

BACKGROUND

The invention relates to devices for generating plasmas by couplingelectromagnetic power into a gas. Such devices are also called “plasmasources”. The terms “plasma generating device” and “plasma source” willbe used interchangeably in the present description.

In order for cold-plasma surface treatment technology to becomecommonplace, there is a need to improve the devices used for generatingthese plasmas by coupling electromagnetic power into a gas. Thesedevices or “plasma sources” must be:

-   -   simple and inexpensive;    -   suitable for linear geometries and possibly for nonplanar        geometries; and    -   capable of operating in a wide range of pressure levels between        a substantial vacuum, of the order of 10⁻² mbar, and atmospheric        pressure or even above atmospheric pressure.

Furthermore, the efficiency in transmitting the electromagnetic powercoming from the generator into the plasma must be as high as possible,i.e.:

-   -   the operation must generate only minimal loss by heating of the        structure of the device for coupling the electromagnetic power        into the plasma;    -   the residual radiation to the outside must be negligible (for        security and for the impossibility of interference with devices        operating in the vicinity at the same prescribed industrial        frequencies); and    -   only a small fraction of the incident power must be reflected        back to the generator, i.e. there must be good impedance        matching between the power supply line and the plasma source        using this same power.

The latter condition must remain true as long as possible for a widerange of operating regimes, without it being necessary to makereadjustments in real time.

Plasmas excited by very high frequencies (especially greater than aroundone hundred megahertz, including microwave frequencies), for example 434MHz, 915 MHz, 2450 MHz and 5850 MHz (frequencies prescribed by theinternational regulations for the IMS (industrial, scientific andmedical) band, are of particular interest because of their high electrondensity. This means a more intense activation of the physico-chemicalprocesses in the discharge, especially a high rate of formation of theactive species involved in a surface treatment process. This treatmentis therefore more comprehensive and/or more rapid: for example, the rateat which materials can be deposited in the form of thin films is higherand the production yield is more favorable.

Above a limit of a few tens of MHz, electromagnetic waves, because oftheir propagation properties, cannot be applied to a gas in order tocreate a plasma by means of electrodes connected to a power supplycircuit, as in the DC or radiofrequency case. The microwaves areconveyed from the generator via a hollow rectangular waveguide or acoaxial cable, before being guided by a conducting structure of aspecific architecture internal or contiguous with the treatment chamber.This chamber must allow distribution and distributed absorption of themicrowaves in order to create a sufficiently uniform plasma with therequired characteristics.

Microwave plasma generator devices have been developed among which thefollowing may be mentioned in particular: the “duo plasmaline” system(E. Rauschle et al. J. de Physique IV (8), PR7, 99 (1998));two-dimensional slotted-antenna applicators (H. Sugai, Plasma FusionResearch 72, 621 (1996) and H. Sugai et al., Plasma Sources Science andTechnology 7, 192 (1998)); microstrip field applicator sources foranalytical applications (A. M. Bilgic et al., Plasma Sources Science andTechnology 9, 1-4 (2000)); electron cyclotron resonance systems; andmulti-dipole magnetron systems.

However, all these devices have a complex architecture and are expensiveto produce. Moreover, they are too dependent on a given configurationand size for the plasma treatment reactor.

SUMMARY

Staying in particular with the studies by the Bilgic et al. team, one ofwhose publications was mentioned above, (the reader may also refer tothe documents DE-198 51 628 and US 2003/008068), these relate to the useof microstrip systems. However, it should be clearly noted as regardsthese studies that the source in question is of very small size and isintended to sustain plasmas with a low power (about 10 W) at atmosphericpressure in argon in a capillary channel (with a cross section of about1 mm²) bored axially in a silica rod of rectangular cross section. Theplasma channel of very small cross section is entirely within amicrostrip transmission line. There is no mention in this work of thepossibility of extending the system to two or three dimensions, and itis difficult therefore to imagine the possibility of using such astructure for the continuous treatment of large areas.

As will be described in greater detail in what follows, with the aid ofcomparative figures, the systems described by Bilgic et al. employ acontinuous conducting plane, kept grounded, on the opposite face of thedielectric, which solution has drawbacks, among which are:

-   -   the coupling into the plasma is rather of the resonant type,        which it is better to avoid since the impedance matching is then        tricky to achieve and is often an unsupportable constraint in        the case of actual practical applications; and    -   in such a configuration, there are not too many ways of        positioning the plasma: a channel may be cut out in the        dielectric, or else the ground plane may be placed at some        distance from the lower face of the dielectric, but in all cases        this distance is limited to a few mm (since the stripline and        the ground plane must “see each other” electrically), which will        in practice considerably limit the applications of such a        configuration.

It will now be explained how we produce, according to the invention,traveling wave propagation and not resonant coupling, and how, when thisis necessary, we eliminate the depth constraint. In particular, we willsee that the present invention can be credited with the idea ofconsidering the plasma as a conductor with an intrinsic potential andtherefore said conductor can pretty well serve as a ground reference,supporting, by itself, the propagation of the traveling wave thatcreates it.

The inventors have found, surprisingly and unexpectedly, that planesources based on microstrip field applicators, and more generally thoseusing an elongate conductor of small cross section compared with itslength (whether of the microstrip type or of the hollow, for example,cylindrical, line type), constitute very simple plasma sources that areeasy to employ and have all of the required qualities.

Thus, the plasma generator device according to the invention comprisesat least one very high-frequency source connected to an elongateconductor of small cross section compared with its length (for exampleof the microstrip type or hollow line type) which is fixed to adielectric support, at least one impedance matching means between thevery high-frequency source and the connection to the conductor, at leastone means for cooling said conductor, and at least one gas feed close tothe dielectric support on the opposite side from the side supporting theconductor.

As will have been understood in reading the foregoing, the expression“very high-frequencies” means according to the invention frequenciesabove 100 MHz, and especially the “discrete” frequencies of 434 MHz, 915MHz, 2450 MHz and 5850 MHz which are prescribed by the internationalregulations for the ISM (industrial, scientific and medical) band.

Likewise, the gas feed being termed “close to” or “in the vicinity of”the dielectric support is understood to mean an inlet typically openingat most 15 mm from the support and preferably at most 10 mm from thesupport.

The plasma is generated below that surface of the dielectric which isopposite the surface supporting the conductor, and facing the latter.Thus, the device according to the invention may be moved with respect tothe surface to be treated in such a way that the plasma is in contactwith this surface to be treated, or else the surface to be treated maybe run beneath the plasma-generating zone, the device according to theinvention then remaining stationary. Depending on the orientation of theconductor relative to the surface to be treated and depending on thedistance separating the dielectric from the surface to be treated, thetreatment will take place directly by the plasma or by thepost-discharge plasma. The term “post-discharge plasma” is understood bythose skilled in the art to mean the region immediately contiguous withthe actual plasma zone, characterized by its intense luminescence. Inthe post-discharge plasma, the charged species have practicallydisappeared, but neutral excited and/or active species still remain.Thus, when the conductor is perpendicular to the surface to be treated,said surface does not encounter the plasma zone and the treatment willtake place by post-discharge plasma, whereas when the conductor isparallel to the surface to be treated (the most common case), thetreatment will take place by direct contact with the plasma.

In the present invention, the term “microstrip” is understood to mean anelectrical conductor element of elongate shape and small thickness,typically of the order of one millimeter or less than one millimeter.The microstrip can have any length and any width, these dimensions beingsuch as to optimize the power propagation properties along thetransmission line formed by the microstrip. As a variant, and as alreadymentioned above, the microstrip may be replaced with a hollow elongateelement, especially one of round, rectangular or square cross section,the wall thickness of the hollow tube being sufficient for goodmechanical strength and having no effect on the electrical behavior. Themicrostrip/conductor is not constrained to a plane, rectilineargeometry, but may also adopt a curved shape in the plane or a warpedshape in its length direction with concave or convex curvatures.

As will have been understood, terms “conductor” and “microstrip” areused interchangeably in what follows, without at any moment the presentinvention being restricted to just one of these types of line.

Because the high-frequency currents flow by obeying the skin effect andbecause this depends on the frequency and the conductivity of thematerial constituting the conductor, the practical thickness in whichthe current flows will be very much less than 0.1 mm. However, becausethe transported power levels are high, of the order of a few hundredwatts, and because the conductivity of the metal decreases withincreasing temperature, the thickness of the microstrip will be verymuch greater than the theoretical thickness defined by the skin effectand it will be necessary to cool the microstrip so that its physicalintegrity is preserved. Thus, the microstrip will have a thickness ofthe order of one millimeter and be made of a material which is a goodelectrical and thermal conductor, both these factors being chosen so asto have good mechanical strength, which may be copper alloys such as,for example, brass or preferably beryllium copper. To maintain the goodconductivity of the microstrip, it may be advantageous to coat thesurface of said microstrip with a coating of a metal which is at leastas good an electrical conductor and insensitive to oxidation (forexample gold). This guarantees that the good electrical characteristicsare maintained over time in a normal operating environment in whichcopper alloys have a tendency to oxidize slightly or to besurface-contaminated.

Advantageously, the microstrip conductor is mechanically pressed againstthe dielectric. It may also be screen-printed on the dielectric if thepower levels involved are low enough.

The dielectric used must have not only good electrical properties, i.e.a low ratio of the imaginary part of its dielectric function to the realpart thereof (i.e. tan δ), typically between 10⁻⁴ and 10⁻², resulting inlow dielectric loss at the operating frequency in question, but alsoexcellent heat shock capability (the thermal gradient due to the plasmain contact with the wall opposite the microstrip may be very high).

Thus, it is possible to choose, as dielectric, either silica, for itsexcellent heat shock resistance, or preferably a ceramic, especiallyboron nitride or aluminum nitride.

Various means for cooling the microstrip may be used. According to afirst embodiment, coolant is made to circulate in an insulating housingplaced on the dielectric and above the microstrip, which coolant iselectrically insulating and has a dielectric constant ε lower than thatof the solid dielectric of the substrate. The coolant must have goodheat-transfer capability. It must also be a good dielectric so asneither to disturb the propagation of the electromagnetic waves alongthe line nor dissipate a substantial fraction of the power byabsorption. The dielectric heat-transfer fluid may for example beadvantageously an α-olefin such as tetradecene (C14). Thus, the deviceaccording to the invention includes a housing placed on the dielectricand on top of the microstrip, confining the circulation of the coolant.

According to a second embodiment, the cooling is carried out indirectlyby placing, over the entire free face of the microstrip, a heat sinkmade of a dielectric, which may be a ceramic, and preferably having goodthermal conductivity (e.g. alumina, or aluminum nitride), in which acoolant circulates. In this case, since the coolant does not circulatein direct contact with the microstrip but at a certain distancetherefrom, it does not circulate in a region of high electromagneticpower density and is not restricted to low absorption of the waves,which fluid may consequently be water.

According to a third embodiment, in the case in which the microstrip isreplaced with a hollow elongate conductor element, a coolant circulatesin the hollow part of said element. The coolant may be water since theelectromagnetic field is zero on the inner wall of the hollow element.This is because the wall thickness of said element is very much greaterthan the skin depth. This solution provides better cooling than thecooling systems described above and enables larger very high-frequencycurrents to flow, and therefore results in higher transmitted powerwithout increasing the electrical losses. The line thus formed with ahollow conductor of rectangular, square or circular cross section can belikened to a hybrid structure from the electrical standpoint incomparison with a plane microstrip line. Experimentally, it has beenconfirmed that this type of line has a characteristic impedancerelatively close to that of a microstrip structure. The fact of nolonger having an intermediate heat sink considerably simplifies thearrangement, and contact between the electrode and the dielectric isprovided by a clamping device identical to the arrangement of a planemicrostrip structure.

According to another embodiment, the device according to the inventionmay also be provided with at least one means for cooling the dielectric.A cooling means may consist of channels provided in the dielectric,through which a coolant circulates. Another means may consist in placingthe dielectric on a support having channels through which a coolantcirculates.

So as not to emit microwaves into the external environment, somethingwhich would be a waste of the power and would create operator safety orelectromagnetic compatibility problems, it is advantageous for themicrowave power coupling device formed by the microstrip line to beenclosed in a conducting housing acting as a Faraday cage.

Depending on the frequency used, the power supply for the devicesaccording to the invention may be transposed directly from the powersemiconductor industry applied to telecommunications. Power generatorsbased on this “solid state” technology are more compact and morereliable than generators based on vacuum tubes, such as magnetronssupplied by a switch mode power supply. Unlike magnetrons, solid-statepower generators require no maintenance, in particular periodicreplacement of a magnetron is eliminated. Furthermore, the cost of thesegenerators drops rapidly with medium-volume and high-volume production.

The microstrip lines may be supplied in various ways:

-   -   in traveling wave mode, by connecting the very high-frequency        wave generator to just one end of the microstrip and connecting        an impedance-matched load to the other end of the microstrip;    -   in traveling wave mode, by connecting a very high-frequency wave        generator to each of the ends of the microstrip in order, on the        one hand, to increase the total power and, on the other hand, to        compensate for the attenuation of the wave by absorption during        its propagation, so as to sustain the plasma. In this case, it        is necessary to use a different generator at each end so that        there is no phase correlation between the two signals, otherwise        a standing wave mode would be established;    -   in standing wave mode, by connecting a very high-frequency wave        generator to only one end of the microstrip and by providing an        adjustable short circuit at the opposite end, in order to        provide impedance matching; and    -   in standing wave mode, by connecting a very high-frequency wave        generator to a divider device, each of the branches of which is        connected to one of the ends of the microstrip.

The lines and connectors are provided by standard commercial components(for example by a coaxial cable having a 50-ohm characteristicimpedance).

The device according to the invention has the additional advantage overwaveguide systems that the impedance matching is also easier to achieve.For example, the conversion and impedance-matching components may beproduced in the form of conventional matching circuits (circuitsconsisting of inductors and capacitors), but also directly in the actualstructure of the microstrip lines by producing therein a quarter-waveimpedance transformer (the principle of which is known to those skilledin the art), or by adding suitable lengths of microstrip (these beingcalled “stubs” in this industry), as propagation line excrescences with,as corollary, integration simplicity, impossibility of detuning (valuesbeing fixed by the geometry and the nature of the dielectric employed)and optimization of the very high-frequency power transfer (lower lossin the connectors and links).

Thus, the impedance matching between the very high-frequency generatorand the microstrip applicator may be achieved by a T or

or L circuit, or by using a stub perpendicular to the microstrip. Theimpedance matching and therefore the dimensions of the stub and themicrostrip are within the competence of a person skilled in the art andmay be determined using a quasistatic analysis in which the startingpoint is the assumption that the propagation mode is exclusively TEM(see the publications by Gupta et al., “Microstrip lines and slot lines”and K. C. Gupta, R. Garg and I. J. Bahl (Hartech House, Norwood, Mass.,1979). In particular, a person skilled in the art would know how toadapt the impedance of the devices in which the microstrip is immersedin a coolant with a dielectric constant greater than 1, or in which adielectric heat sink of dielectric constant greater than 1 is pressedagainst the microstrip.

In order for a larger area to be treated simultaneously and uniformly,it is advantageous to combine several devices according to theinvention. By juxtaposing a plurality of plasma generator devices ispossible in fact to generate a plasma sheet over large areas, which inall events applies to continuous treatment on the run.

It is possible to combine as many elements as are needed to carry out acontinuous surface treatment with the desired production yield. Each ofthe plasma generator devices thus combined includes at least one veryhigh-frequency source connected via an impedance matching system througha microstrip conductor fixed to a dielectric support, at least one meansfor cooling said microstrip and at least one gas feed close to thedielectric support on the opposite side from the side supporting themicrostrip.

For surface treatment applications operating at atmospheric pressurewith the need to run the substrate beneath the active zone, it ispossible to conceive of various arrangements of plasma modules that canbe easily integrated, while still benefitting from the inherentsimplicity of this type of source.

The plasma generator devices may be placed end to end so as to cover thewidth of the substrate or may be offset in the run direction so as tooverlap the area to be treated. It is also possible to add the plasmagenerator devices in the run direction so as, if necessary, to increasethe time in contact with the active zone, depending on the run speed, inparticular so as to increase the productivity.

The assembly consisting of the various devices may be joined together bymeans of a common base or mechanical structure which fulfils the gasdelivery and cooling functions and the electromagnetic powerconnections.

Advantageously, the connections may be very limited, by connecting theamplifier module of the very high-frequency power generator, togetherwith its integrated impedance matching device, directly to themicrostrip.

The assembly consisting of various plasma generator devices joinedtogether by means of a base or mechanical structure, which fulfils thegas delivery and cooling functions and the electromagnetic connections,has in particular the following advantages:

-   -   it is simple to produce and to integrate, thereby making mass        production possible and limiting the manufacturing costs, and        making maintenance easy;    -   by reducing the electrical connection to a single connector (not        a coaxial cable), the losses in transporting the power to the        plasma module are reduced, this having an important impact on        the design and therefore the cost of the very high-frequency        part.

Furthermore, with the devices according to the invention, it is possibleto use plasma module excitation frequencies a little lower than those inthe microwave range, such as for example 434 MHz (ISM band), making itpossible to benefit from the all-semiconductor technology with a goodyield.

Another subject of the invention relates to modular small-sizedmoderate-power plasma torches that also benefit from the same advantagesas those described above. These plasma torches have the samearrangements and forms (microstrip/flat or hollow conductor) as theabove applicators. More particularly, a longitudinal channel passesright through the dielectric on which the conductor is placed. Gas isinjected via one of the ends, and the plasma forms in the channel,extending over the entire length thereof. By varying the gas flow rateand the very high-frequency power, it is possible either to extract theplasma at the end of the torch or to use the post-discharge plasmathereby moving the substrate to be treated further way. The crosssection of the channel may of course be optimized so as to confine theplasma.

Thus, a plasma torch according to the invention comprises at least onevery high-frequency source with its integrated impedance matching deviceconnected to a conductor (for example of the microstrip type or hollowconductor type) fixed to a dielectric support and at least one means forcooling said conductor, said dielectric support being longitudinallypenetrated by a channel via one end of which the gas is injected and inwhich the plasma forms.

Because of its simple design, it is possible to use this type of plasmatorch on a robot arm so as to apply the plasma treatment by scanning asurface to be treated.

According to one of the aspects of the invention, the device accordingto the invention, and contrary to what the prior art recommends (i.e.the presence of a ground plane extending at least facing the entiresurface of the conducting transmission line, on the opposite surface ofthe dielectric) the device according to the invention therefore includesa ground plane, but this is in no case continuous, only a minor area ofthe transmission line (microstrip or conductor) facing a ground plane.

This aspect of the invention will be described in conjunction with theappended FIGS. 14, 15 and 16 which illustrate the case in which anelongate conductor of the microstrip type is used.

FIG. 14 illustrates the case of the prior art involving in particularthe work by the Bilgic et al. team. The structure is made up of amicrostrip and a complete continuous ground plane, these being separatedby the dielectric substrate. In this case, as already mentioned, it isimplicitly impossible to sustain the plasma beyond the geometricalboundary formed by this complete continuous ground plane, for example totreat a substrate placed in an extended chamber located thereunder. Infact, another useful configuration could be used, noting that amicrowave edge field extends into the space from the lateral slotsdefined between the edges of the microstrip and the ground plane. Ofcourse if there is no field confinement in the nearby zone lying abovethe microstrip conductor and the dielectric, then it is possible, as analternative, to create an extended plasma in this zone (by optionallyproviding a dielectric superstrate between which and the substrate themicrostrip conductor line is sandwiched, said substrate then being ableto constitute the window of a treatment chamber. However, thisarrangement would hardly be advantageous since, on the one hand, it ismore complex and, on the other hand, the plasma can be sustained only bythe edge fields leaking through the slots defined between microstripconductor(s) and ground plane, and therefore in a spatiallydiscontinuous manner. In particular at atmospheric pressure, this is avery serious drawback since, owing to the short mean free path, it willbe very difficult to make the plasma homogeneous so as to be useful inpractice. The width of the microstrip conductor and the thickness of thesubstrate are small compared with the wavelength in free space. The modeof propagation along such a line is to a first approximation the TEMmode. However, embodiments would also be conceivable in which the activeconducting parts are instead in the form of rectangles. However, thisarrangement is not a priori more advantageous than the previous one.

At the end of the day, a configuration (of the Bilgic et al. or othertype) in which a complete continuous ground plane exists appears to befraught with particularly unacceptable drawbacks.

As also mentioned earlier, the present invention can be credited withhaving thought of considering the plasma sheet as a conductor with anintrinsic potential, which therefore can serve perfectly as a groundreference. The arrangement shown in FIG. 15 is then obtained. In thiscase, the field wave also extends into the plasma. To “launch” such awave, a suitable distribution of the field in the straight section ofthe propagation line must be imposed at the start of the line.

The present invention thus provides a partial metal ground plane at thestart of the line (at the point where the microwaves enter), which willsuffice for launching and sustaining the propagation of the travelingwave and for sustaining a continuous plasma over the entire length ofthe line, facing the latter and beneath the dielectric.

More generally, according to one of the embodiments of the invention, aground plane fraction is used, but its projection normal to thepropagation line intercepts a minor area of the section of the line.

The appended FIGS. 16-a) and 16-b) therefore illustrate two embodimentsof the invention.

The wave launch zone, at the inlet of the transmission line, has aconventional structure, with the microstrip, a metal ground plane andthe dielectric wall of the treatment chamber serving as substrate. Themetal ground plane is interrupted a short distance from the entry and isreplaced with the plasma extending with the microstrip over the entireremainder of the length of the conductor line (FIG. 16-a)).

But it is also possible, because the interface between a dielectric walland a plasma sheet can form a guiding structure for an electromagneticwave, as an alternative, to dispense with extending the microstripsubstantially beyond the boundary of the metal ground plane (FIG.16-b)). In this case, the analog of a device and of a surface waveplasma mode, but in a plane geometry, is then obtained.

The partial surface of the microstrip facing which is a ground planefraction may not be solely at the start of the line (end edge) but mayalso take the form of an overlap of the lateral edges of the microstripwith a ground plane boundary line. For example, a window substantiallymatching the shape of the microstrip, but slightly smaller, may be openin the ground plane surface.

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the invention will now be explained indetail with the aid of the appended drawings in which:

FIG. 1 a shows front view of an embodiment of the device according tothe invention, in which the microstrip is plane but of curved shape,enabling a nonplanar surface to be treated by post-discharge plasma;

FIG. 1 b shows side view of an embodiment of the device according to theinvention, in which the microstrip is plane but of curved shape,enabling a nonplanar surface to be treated by post-discharge plasma;

FIG. 2 a shows front view of an embodiment of the device according tothe invention in which the microstrip is of warped shape, enabling anonplanar surface of a substrate to be directly treated in the plasma;

FIG. 2 b shows side view of an embodiment of the device according to theinvention in which the microstrip is of warped shape, enabling anonplanar surface of a substrate to be directly treated in the plasma;

FIG. 3 a shows a schematically various connection of the microstripconductor to the very high-frequency generator;

FIG. 3 b shows a schematically various connection of the microstripconductor to the very high-frequency generator;

FIG. 3 c shows a schematically various connection of the microstripconductor to the very high-frequency generator;

FIG. 3 d shows a schematically various connection of the microstripconductor to the very high-frequency generator;

FIG. 4 a shows schematically possible way of matching the impedance ofthe device;

FIG. 4 b shows schematically possible way of matching the impedance ofthe device;

FIG. 4 c shows schematically possible way of matching the impedance ofthe device;

FIG. 5 shows, in cross section, a device according to the invention witha plane microstrip provided with a first embodiment of the coolingmeans;

FIG. 6 shows, in cross section, a device according to the invention witha plane microstrip provided with a second embodiment of the coolingmeans;

FIG. 7 shows, in cross section, a device according to a secondembodiment of the invention with a propagation line element of hollowcross section, this being an alternative to the microstrip;

FIG. 8 shows, in cross section, a device according to a secondembodiment of the invention with a propagation line element of hollowcross section, this being an alternative to the microstrip;

FIG. 9 a is a representation, in longitudinal section, of a deviceaccording to the invention, provided with a plane microstrip;

FIG. 9 b is a representation, in cross section, of a device according tothe invention, provided with a plane microstrip;

FIG. 10 a is a representation, in longitudinal section, of a deviceaccording to the invention provided with a propagation line element ofhollow cross section, this being an alternative to the microstrip;

FIG. 10 b is a representation, in cross section, of a device accordingto the invention provided with a propagation line element of hollowcross section, this being an alternative to the microstrip;

FIG. 11 shows, in cross section, an assembly of devices according to theinvention;

FIG. 12 shows, in cross section, another assembly of devices accordingto the invention; and

FIG. 13 a shows longitudinal of a plasma torch employing a deviceaccording to the invention.

FIG. 13 b shows cross section of a plasma torch employing a deviceaccording to the invention;

FIG. 14 shows the case of the prior art involving in particular the workby the Bilgic et al. team. The structure is made up of a microstrip anda complete continuous ground plane, these being separated by thedielectric substrate;

FIG. 15 shows an embodiment that considers the plasma sheet as aconductor with an intrinsic potential, which therefore can serveperfectly as a ground reference. In this case, the field wave alsoextends into the plasma;

FIG. 16 a shows an embodiment of the invention with a wave launch zone,at the inlet of the transmission line, that has a conventionalstructure, with the microstrip, a metal ground plane and the dielectricwall of the treatment chamber serving as substrate. The metal groundplane is interrupted a short distance from the entry and is replacedwith the plasma extending with the microstrip over the entire remainderof the length of the conductor line; and

FIG. 16 b shows an embodiment of the invention that dispenses withextending the microstrip substantially beyond the boundary of the metalground plane.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 a and 1 b illustrate schematically a device 1 according to theinvention, in which the microstrip 2, which has a plane but curvedshape, is connected to a very high-frequency generator. This microstrip2 is fixed to the surface of a dielectric support 3B, one edge of whichcoincides with one of the curved edges of the microstrip. Provided inthe dielectric is a slot 4 into which the gas is injected and in whichthe plasma 5 is generated. A substrate 6 to be treated, on averageperpendicular to the plane of the microstrip and having a warped shapematching the curvature of the dielectric and of the microstrip, isdriven beneath the device in the direction indicated by the arrow.According to this embodiment, the substrate is perpendicular to themicrostrip, the treatment is a post-discharge plasma treatment.

FIGS. 2 a and 2 b illustrate schematically a device 7 according to theinvention, in which the microstrip 8 of warped shape is connected to avery high-frequency generator. This microstrip 8 is fixed to the actualwarped surface of a dielectric 9B. The gas is fed in close to the face9A.2 of the dielectric 9A.1-A.2 and the plasma 10 is generated beneaththe face 9A.2 opposite the microstrip 8. A substrate 11 to be treated,having a warped shape matching that of the dielectric 9B and of themicrostrip 8, is driven beneath the device 7 in the direction indicatedby the arrow. In this embodiment, since the substrate 11 isperpendicular to the microstrip, the treatment is a direct plasmatreatment.

FIGS. 3 a to 3 d show schematically the various ways of connecting themicrostrip conductor to the very high-frequency power supply. Thus,according to a first embodiment (FIG. 3 a), the microstrip 12 issupplied so as to propagate a traveling wave along the microstrip. Thevery high-frequency range generator is connected via a coaxial line, forexample having a characteristic impedance of 50Ω (this value generallycorresponding to the industrial standard) at only one end 12 a of themicrostrip 12, for the other end 12 b being connected to a matchedimpedance load 14, that is to say there is no reflection of the waves atsaid end opposite the connection to the generator and therefore nostanding wave along the microstrip. In this embodiment, the intensity ofthe wave decreases very substantially along the microstrip, owing to thegradual absorption of the power in order to sustain the plasma.Therefore, the latter is not very uniform along the microstrip.

According to a second embodiment, illustrated in FIG. 3 b, themicrostrip 15 is supplied so as to propagate two opposed traveling wavesstarting from each of its ends, so that their intensities add together.To do this, one end 15 a of the microstrip is connected via a coaxialline 17 to a first very high-frequency wave generator 16 and theopposite end 15 b of the microstrip is connected via a coaxial line 18to a second very high-frequency wave generator 19. Since the phases ofthe signals of two separate generators are uncorrelated, it is theintensities of the two counter-propagating waves that add together, andnot their amplitudes (this would result in the appearance, throughinterference, of a standing wave), partly compensating for the observedgradient with a single source at one end.

According to a third embodiment illustrated by FIG. 3 c, the microstrip20 is supplied so as to create a standing wave mode along themicrostrip. One end 20 a of the microstrip 20 is connected via a coaxialline 21 to a very high-frequency generator. A short-circuit device 22 isconnected to the other end 20 b. This short-circuit device 22 isadjustable, so as to vary the complex reflection coefficient and matchthe impedance so as to optimize the characteristics of the standingwave.

According to a fourth embodiment illustrated by FIG. 3 d, the microstrip23 is supplied so as to create a standing wave mode along themicrostrip. A very high-frequency generator is connected via a coaxialline 24 to a power divider device 25 (standard industrial equipmentknown to those skilled in the art), each of the branches 26 a and 26 bof which is connected to one end 23 a and 23 b of the microstrip 23.Since the phases of the waves coming from the same generator arecorrelated, it is clearly the amplitudes of the waves that add together,and not their intensities, giving rise by interference to a standingwave. As power divider, it is possible for example to use aWilkinson-type device known in the literature.

FIGS. 4 a to 4 c show schematically three impedance matching modes.

Thus, in FIG. 4 a, the very high-frequency generator is connected to themicrostrip 27 via an impedance matching circuit which in this particularcase is a T-network 28. In FIG. 4 b, the very high-frequency generatoris connected directly to the microstrip 29 on that side where the latteris provided with a microstrip stub 30 of length L and width W, the stubbeing perpendicular to the microstrip 29. By choosing the geometricparameters L and W it is possible to modify the electrical effect of thestub and thus apply the desired correction to the resulting impedance ofthe system. In FIG. 4 c, the very high-frequency generator is connectedto the microstrip 31 via a quarter-wave impedance transformer producedin the microstrip 32 lying in the longitudinal extension of the mainmicrostrip and having an effective electrical length of λ/4, λ being thewavelength for propagation along the microstrip line attached to thesubstrate of a given dielectric constant, at the very high-frequency inquestion. The function of the quarter-wave impedance transformer is toenable the incident power coming from the generator to “see” aneffective impedance equal to the characteristic impedance of the mainmicrostrip line forming the field applicator, the plasma being ignited(the microstrip/plasma assembly constituting a complex load). Thegeneral rule in designing a quarter-wave impedance transformer on atransmission line is well known. If Z_(C) is the output impedance of thegenerator and Z_(L) is the characteristic impedance of the microstripline (with the plasma ignited), the impedance Z_(t) of the quarter-wavetransformer will be z_(t)=√{square root over (Z_(C)Z_(L))}.

FIG. 5 shows, in cross section, a device 33 according to the inventionthat comprises a microstrip 34 fixed to a dielectric which is aparallelepipedal element having an elongate recess forming a channel 36and placed on a support 37 made of a conducting material, forming anelectrical reference plane, penetrated over its entire height by a slot38 and, on either side of said slot, by longitudinal slots 39 a and 39 bthat are symmetrical with respect to the slot 38 and via which the gasis supplied. The conducting support 37 acts as a partial ground plane asdefined above, the slot 38 being narrower and shorter than themicrostrip 34 so that there is a conducting ground plane fraction facingthe ends of the microstrip and opposite the lateral edges of saidmicrostrip over its entire length. Fixed to the upper face of thedielectric 35 a supporting the microstrip 34 is a housing 40 made of adielectric material, in which housing a dielectric coolant 41circulates, the entire microstrip 34 being in contact with the coolant41. A Faraday cage 42 encloses the dielectric 35 and the housing forconfining the coolant 40. The plasma 43 is generated in the channel 36and the active species escape via the slot 38 in the direction of thearrow, because they are entrained by the gas stream.

FIG. 6 shows, in cross section, a device 44 according to the inventionthat differs from the embodiment shown in FIG. 5 by the fact that theinsulating housing containing a coolant in contact with the microstripis replaced with a heat sink 45, which is a parallelepiped made of adielectric material pressed against the upper face surface (on theopposite side from the substrate and from the plasma) of the microstrip34 and penetrated by a channel 47 in which a coolant 48 circulates,which is no longer necessarily a very good dielectric at the very highfrequency in question, but may for example be water.

FIG. 7 shows, in cross section, a device 49 according to the inventionthat differs from the embodiment shown in FIG. 6 by the fact that themicrostrip 34 and the dielectric heat sink 45 have been replaced with atransmission line element 50 which is a hollow conductor element ofcircular cross section in which a coolant 51 circulates. Of course, thesurface 35 a of the dielectric 35 has been modified in order to matchthe shape of the conductor element 50.

FIG. 8 shows, in cross section, a device 52 according to the inventionthat differs from the embodiment shown in FIG. 7 by the fact that thetransmission line element 53 is a hollow conductor of rectangular crosssection in which a coolant 51 circulates. The surface 35 a of thedielectric 35 is then plane, as in the case of the embodiments shown inFIGS. 5 and 6.

A plasma generator device 54 provided with a cooling system such as thatof FIG. 6 is shown completely in FIGS. 9 a and 9 b. This device 54 ismade up of the following various elements stacked one on top of another:

-   -   a base 55 penetrated by two symmetrical longitudinal channels 56        a and 56 b in which water circulates and by two symmetrical        channels 57 a and 57 b for delivering the gas entering the        discharge with, at the center, an output slot 58 for extracting        the active species from the plasma 59, it being necessary to        cool the base because of the heat released by the plasma, which        is in contact with the dielectric substrate;    -   a dielectric 60 forming, above said slot 58, a channel 61 of the        same width as the microstrip 62 and with the same length;    -   said microstrip 62 consists of a conducting metal strip        connected to the connector for transmitting the very        high-frequency power coming from the generator, and being fixed        above said dielectric 60; and    -   a ceramic dielectric heat sink 63 having a longitudinal channel        64 in which water circulates, said heat sink 63 being pressed        against the entire surface of the microstrip 62.

A clamping system 9 as illustrated in FIGS. 9 a and 9 b, for clampingthe stack, enables the elements to be pressed against and held in placeon the base 55. An O-ring seal (not shown) located in the lower partseals the volume in which the discharge develops.

The entire device is confined in a conducting housing 66 acting as aFaraday cage so as to avoid any leakage of radiation to the externalenvironment, which would have associated safety and electromagneticcompatibility problems.

A plasma generator device 67 provided with a cooling system such as thatof FIG. 7 is shown completely in FIGS. 10 a and 10 b.

This device 67 differs from that of FIGS. 9 a and 9 b by the fact thatthe microstrip 62/insulating heat sink 63 assembly is replaced with alongitudinal transmission line element of hollow circular cross sectionin which water circulates. The transmission line element is held inplaced by a dielectric spacer inserted into the rest of the stack andimmobilized by clamping means 70.

FIG. 11 shows an assembly 71 of three plasma generator devices (given asan example, it being possible for this number to be increased withoutany particular limit), each comprising a very high-frequency supplymodule 72 for supplying a microstrip conductor 73 with veryhigh-frequency power. The microstrip is cooled by means of a dielectricheat sink 74, through the internal channel 75 of which water circulates.The microstrip is fixed to a dielectric substrate 76. The various units,each comprising a microstrip, dielectric, very high-frequency supply anddielectric heat sink, are held together by a distribution blockincorporating gas supply lines 79 and cooling water supply lines 80. Theplasma 81 is generated on the lower face of the dielectric substratefacing the microstrip. The substrate 82 to be treated runs beneath eachof the plasma sources. If the substrate 82 is conducting, for example ifa steel or aluminum sheet is to be treated, said substrate acts asground plane. If the substrate is a dielectric, a ground plane fraction(not shown) must be provided beneath the dielectric box 76, for examplea plane conducting element extending over a limited distance from thatend of the microstrip supplied with power in the direction perpendicularto the plane of the figure (generic arrangement of FIG. 16).

FIG. 12 shows another type of assembly 83 comprising two dielectric84/microstrip 85 units (this number of units not being limiting)enabling a plasma 86 to form in the slot 87 supplied with gas via thegas inlet 88. The gas is then entrained toward the gas outlet 89. Themicrostrip is cooled by circulation of a dielectric coolant in thechannel 90 surrounding the microstrip. The distribution block 91 iscooled by water circulating in channels 92. According to the generalprinciple of the invention, to maintain the plasma as potentialreference and to avoid a resonant system, the ground blocks defining theslots 87 facing the microstrips 85 will be made of a conducting materialonly over a limited length starting from that end of the microstripsupplied with power, it being possible for the rest of the total lengthof the block (in the direction perpendicular to the plane of the figure)to consist of a dielectric rod.

FIG. 13 shows a plasma torch 93 comprising a base 94 incorporating acoaxial longitudinal channel 95 which is closed at one end and in whichwater circulates, with an inlet and an outlet at the other end. Placedabove this base 94 is a dielectric 96 penetrated right through by alongitudinal channel 97 into which the gas is injected and in which theplasma 98 is generated. The microstrip 99 connected to the veryhigh-frequency generator is fixed above the dielectric. Placed on thefree face of the microstrip 99 is a dielectric heat sink in which water101 circulates. The assembly is inserted into a Faraday cage 102.

It will be understood that many additional changes in the details,materials, steps and arrangement of parts, which have been hereindescribed in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims. Thus, the presentinvention is not intended to be limited to the specific embodiments inthe examples given above.

What is claimed is:
 1. A plasma generator device which comprises atleast one source of power with a frequency above 100 MHz, said sourcebeing connected via an impedance matching system to an elongateconductor fixed in intimate contact over its entire lower surface to adielectric support, at least one means for cooling said conductor and atleast one gas feed close to the dielectric support on the opposite sidefrom the side supporting the conductor, wherein said device includes apartial electric ground plane that lies facing a face of the dielectricon the opposite side from the side supporting the conductor, the partialcharacter of the ground plane being expressed by the fact that only aminor area of the conductor line is facing a ground plane.
 2. The deviceof claim 1, wherein the conductor has a thickness of the order of onemillimeter.
 3. The device of claim 1, wherein the conductor is amicrostrip.
 4. The device of claim 1, wherein the conductor is a hollowelongate element, especially of round, rectangular or square crosssection.
 5. The device of claim 1, wherein the partial ground plane islocated at the start of the conductor line, the point where themicrowaves enter the device.
 6. The device of claim 5, wherein the wavelaunch zone, at the input of the conductor line, has a conventionalstructure in which the elongate conductor, the dielectric and thepartial ground plane are assembled, the ground plane being interruptedat a short distance from the input of the conductor line and then beingreplaced with the plasma extending with the conductor over the entirerest of the length of the conductor line.
 7. The device of claim 5,wherein the wave launch zone, at the input of the conductor line, has aconventional structure in which the elongate conductor, the dielectricand the partial ground plane are assembled, the ground plane beinginterrupted at a short distance from the input of the conductor line andthen being replaced with the plasma, the conductor not extendingsubstantially beyond the boundary of the ground plane.
 8. The device ofclaim 1, wherein the conductor is made of a copper alloy chosen from thegroup comprising brass and, preferably, beryllium copper.
 9. The deviceof claim 1, wherein the conductor is mechanically fixed to thedielectric.
 10. The device of claim 1, wherein the conductor isscreen-printed onto the dielectric.
 11. The device of claim 1, whereinthe dielectric has a dielectric loss tangent tan δ of between 10⁻⁴ and10⁻².
 12. The device of claim 1, wherein the dielectric is silica or aceramic, preferably aluminum nitride or boron nitride.
 13. The device ofclaim 1, wherein the device is placed in a conducting housing acting asa Faraday cage.
 14. The device of claim 1, wherein a dielectric housingis placed on the dielectric substrate of the conductor line and abovethe conductor, and in that a coolant of low dielectric loss circulatesin said housing.
 15. The device of claim 1, wherein a heat sink made ofa dielectric material, through which a coolant flows, is placed over theentire free face of the conductor.
 16. The device of claim 1, whereinthe elongate conductor is a hollow longitudinal conductor provided ateach of its ends with an opening for the circulation of a coolant. 17.The device of claim 1, wherein it includes means for cooling thedielectric substrate.
 18. The device of claim 17, wherein the dielectrichas channels in which a coolant circulates, or in that the dielectric isplaced on a support having channels in which a coolant circulates. 19.The device of claim 1, wherein the surface of the conductor is coatedwith a coating of a metal which is a good electrical conductor and isresistant to oxidation.
 20. The device of claim 1, wherein saidimpedance matching system is produced from impedance matching componentsproduced in the actual structure of the conductor.
 21. A plasmagenerator device, comprising at least two of the devices of claim
 1. 22.A plasma torch comprising at least one very high-frequency sourceconnected via an impedance matching device to an elongate conductor,fixed to a dielectric support, and at least one means for cooling saidconductor, said dielectric support being longitudinally penetrated by achannel via one end of which the gas is injected and in which the plasmaforms, the active species of said plasma being extracted by the gas flowvia the opposite end, wherein said device includes a partial electricground plane that lies facing a face of the dielectric on the oppositeside from the side supporting the conductor, the partial character ofthe ground plane being expressed by the fact that only a minor area ofthe conductor line is facing a ground plane.